Multi projection (MP) systems are attracting attention to provide very high precision and high quality image on large display area. Such MP system might be employed for large projection areas like dome, stadium and concert hall or for projection on building.
Each single video projector (VP) of a MP system generates an image with a definition and a size determined by the VP lens focal length, the size of the VP's light modulation device (e.g. a LCD panel) and the distance between the VP and the screen or display zone. Covering a large projection screen with a sufficient definition and brightness usually requires aggregating several VPs in a manner that they cover adjacent, partially overlapping zones of the total screen area. In the overlapping zones, blending ensures a smooth transition among different VPs so as to be tolerant against small displacements introduced e.g. by vibrations or thermal expansion. VPs are commonly equipped with zoom lenses (i.e. lenses with variable focal length) to provide the user some freedom when installing the VP, as for example for adapting to the distance between the VP and the screen.
Usually a MP system includes a wired communication network to distribute video image data from a video source device to the VPs. The video image is distributed to the VPs through the wired communication network. Such a MP system is illustrated in US2008100805 which discloses an asynchronous, distributed, and calibrated apparatus providing a composite display from a plurality of plug-and-play projectors. The apparatus comprises a plurality of self-sufficient modules. Each module comprises a plug-and-play projector. A camera is coupled to each projector. A software or firmware controlled, computation and communication circuit is coupled to the projector and executes a single-program-multiple-data (SPMD) calibration algorithm that simultaneously runs on each self-sufficient module to generate a scalable and reconfigurable composite display without any need for user input or a central server.
The drawback of such system is that it sometimes requires long and costly cables to connect the video source to each VP. For very long distances, some repeaters shall be added to guarantee correct signals shape at VP input connector. Also especially for outdoor installation, it may be difficult even impossible to install several cables from the video source to each projector. To mitigate the burden of cabling operation, one solution is to connect the video source to only one video projector and then to interconnect each VP through a daisy chain wired structure. This is illustrated in the U.S. Pat. No. 7,061,476 where an image to be displayed is transferred from the video source to a first VP, and then from first VP to a second VP and so on. Here again specific cabling is required, with potentially long cables if VPs are far from each other.
From the above examples, it appears that wireless connectivity can solve some cabling issue in MP system. As the performance of wireless technology is improving in terms of throughput, it becomes conceivable to wirelessly interconnect VPs within a MP system, even for the transmission of uncompressed video. The advantage of transferring uncompressed video is to benefit from the highest quality as there is no compression, and to provide a very low latency system allowing interactivity with the user (for instance for simulation tool). Transferring uncompressed video requires large bandwidth, in the order of several Gbps (Giga bits per second), but it becomes achievable with most recent wireless technologies like 60 GHz millimeter wave operating in the 57-66 GHz unlicensed spectrum.
60 GHz-based communication systems are widely studied (e.g. IEEE 802.11ad Task Group; IEEE 802.15.3c standard; Wireless HD; WiGig) and the research community proposes several solutions and methods to transport the audio and video applications with a desired quality of service.
In a wireless communication system, connection setup and communication link bandwidth reservation are conducted before transmitting a video stream. Ideally, sufficient link bandwidth can be allocated and the video stream can be transmitted smoothly after stream set-up control.
An approach of wireless VP is illustrated in the publication US20040217948, depicting a 60 GHz millimeter wave connection between a laptop and a single VP. Also the publication US20100045594 describes a system made of several displays connected to a video source and a control device through wireless connections. In this latter system, there are no wireless transmissions of video data between displays; video data are only transmitted by the video source. This solution is not adapted in the case of MP system, as some VPs may be not visible from the video source, or the quality of communications may be poor or subject to masking conditions. Indeed, video source is usually located at the ground level while VPs are typically hanging overhead or to a metallic structure so that there are no obstacles between VPs and the display screen. As a consequence the conditions of wireless communications between VPs are likely to be safe, at least with better robustness than the wireless communication between a VP and a video source. For this reason, it is advantageous to consider transmitting all video data from the video source to one of the VP of the MP system, and then to wirelessly distribute video data from this particular VP to the other VPs.
There are various ways to implement the wireless function according to some dimensioning parameters or constraints. First of all, the video resolution defines the required bandwidth for data transmission. Let's consider a cluster of 4 VPs to display a 3840×2160 pixel uncompressed video source with 60 frames per second and 4:2:0 chrominance sub-sampling (i.e. average of 12 bits per pixel). Taking into account that a video image is split into 4 image sub-parts with, for instance, 20% of overlapping zones for blending function, the VP connected to the video source, also called the master VP, receives all video image sub-parts at a bit rate of 7.16 Gbps. Then, the master VP shall keep one image sub-part for display and it shall transmit the 3 other image sub-parts to the other projectors. Therefore, the minimum bandwidth required on the wireless medium is 5.37 Gbps (3×1.79 Gbps). Based on specifications of current emerging standards for 60 GHz millimeter wave domain, up to 4 radio channels can be simultaneously used with typical useful throughput of around 3.8 Gbps per radio channel (after demodulation and error correction code). For instance, such figure corresponds to the HRP mode 2 of IEEE 802.15.3c AV mode standard specifications. However this figure shall be lower to around 3.5 Gbps to take into account the overhead due to inter-frame gaps, transmission of preambles and MAC (Medium Access Control) headers. According to the above example, a way to transmit video image sub-parts from one master VP to 3 VPs would be to install three radio modules on master VP operating on 3 different radio channels, and to install one radio module on each other VP which supposes providing three independent point-to-point transmissions. However such a solution would appear costly. Indeed, it would be advantageous to deliver all projectors with identical hardware configuration, letting the user free to install VPs in a MP system or just to benefit from wireless connectivity in a single VP configuration.
A further question is how to place the antenna of each radio module so that VPs can efficiently communicate in different MP system configuration (generally square or rectangular shape). Here, it may be proposed to use smart antennas that allow controlling the direction of antenna radiation pattern. The smart antennas are made of a network of radiating elements distributed in a matrix form on a support. These types of antennas allow the implementation of the technique known as “Beam Forming”. According to this technique, each radiating element of the antenna is electronically controlled in phase and power to obtain a directional transmission and/or directional reception beam. As this technique involves additional complexity for control and additional cost for the antenna itself, it is preferable to consider static antenna with a quasi omni-directional radiation pattern. In case of long distance between VPs, then smart antennas providing beam forming technique would be chosen but here again it is advantageous to limit their numbers for cost reason.
A goal of certain aspects of the present invention is to provide a VP with limited hardware cost for the wireless communication means allowing efficient wireless video data distribution within a cluster of VPs forming a MP system.
Another goal of aspects of the invention is to facilitate the set-up of MP system and make it more flexible.
It is another goal of aspects of the invention to restrict the number of occupied radio channels as the number of available radio channels can be limited due to regulations restrictions or to the presence of interference.
A still further goal aspects of is to efficiently and reliably control the image distribution and the image projection timing in a MP system.